wrote in message
ups.com...
Answering Matt's statement that vapor lock has nothing to do with the
type of fuel: One reason why the FAA is reluctant to grant STC's for
mogas is because of its vapor-lock propensities. There have been
studies and plenty of discussion about the differences in vapor
pressure at the same temperature and pressure between avgas and mogas.
There was some discussion not long ago, iirc, in RAH. Google and you
will find.



http://www.avweb.com/news/maint/187232-1.html

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A large proportion of low-compression aircraft engines from both Lycoming
and Continental were originally certificated for operation on 80/87 octane
aviation gas. Most Lycoming O-235, O-290 and O-320 engines fall in this
category, and so do some of the larger O-360 and O-540 engines. Most
Continental O-200, O-300 and O-470 engines, and some of the fuel-injected
IO-470 and IO-520 engines can run it as well.
So, if you have a low-compression engine, can you just fill it up with
autogas and take off? Nope, you've got to get an appropriate STC -- and
despite what you may have heard elsewhere, it is very important to get that
STC, even though it usually will consist of one or two pieces of paper, plus
new decals for your fuel ports.

Why is the STC important? While unleaded autogas provides sufficient octane
to substitute for 80/87 avgas in low-compression engines, there are other
differences that can cause problems when using autogas in some engine
installations. The two most significant are lower vapor pressure -- which
can lead to vapor lock -- and incompatibility between some of the additives
in autogas and some components (particularly seals) in some aircraft fuel
systems.

In order to qualify for an STC, a particular airframe/engine combination has
to be rigorously tested, to include either a 150 hour engine endurance test
or 500 hour flight test, under controlled conditions. The tests also include
checking operation at high ambient temperatures, which can create vapor
lock. Some aircraft don't pass -- the Piper Apache and Comanche-250, and
Cessna Skyhawk with Avcon's 180HP conversion all failed testing, and cannot
legally run autogas.

In a nutshell, by buying the STC you are paying for a bunch of research and
testing to verify that it really is safe to use autogas in the
airframe/engine combination you have. In a few cases, you may be required to
have modifications made or the STC may authorize only premium (91 octane or
higher) autogas. For example, Petersen Aviation's STC for Piper
PA-28-160, -161, -180, and -181 models requires replacing the electric boost
pump and running premium gas.

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